In calibrating respiratory gas analysis instruments, it has long been the practice to supply the instrument with a pulsatile flow of calibration gas (usually air) from a device known as a calibration syringe. This syringe typically includes a piston and cylinder arrangement which pumps gas into the instrument, through a check valve, as the operator moves the piston between first and second positions. Because the cylinder has a volume comparable to the volume of gas that is exhaled during a typical human breath, and because the ejection stroke of the piston has approximately the same duration as an exhaled breath, the calibration syringe allows the instrument to be calibrated under conditions that simulate those which exist when the instrument is later used with a test subject.
The use of known volume calibration syringes and procedures has been found to result in sizable errors in the volume of gas delivered during calibration. One cause of this error, known as "siphoning", results from the fact that the inertia of the gas flowing through the check valve has a tendency to open the check valve when the piston reaches the end of its stroke. Such siphoning affects the accuracy of the calibration process by causing the actual volume of gas supplied to the instrument to exceed the volume of the calibration syringe.
Another error that is associated with the use of manually operated calibration syringes results from the fact that, due to operator inattention, the piston may not be moved between exactly the same beginning and end positions during each ejection stroke. An operator may, for example, not withdraw the piston to its true outermost position, or may not push the piston to its true innermost position. Any such deviations from the desired inner and outer positions affect the volume of gas delivered by the syringe during calibration and, therefore, the accuracy of all measurements that are based on that calibration.
Another even larger error that is associated with the use of manually operated calibration syringes is the error that results from the nonlinearity of the response of the gas turbine. This nonlinearity can cause the number of output pulses that are produced by the turbine during the flow of a known volume of calibration gas to vary substantially, depending upon the rate at which the gas is delivered. The difficulty is that most operators have difficulty in operating the syringe in a consistent manner. As a result the number of turbine output pulses produced during an ejection stroke of the syringe will vary randomly from stroke to stroke. In the past this nonlinearity has been dealt with by introducing a flow of a "bias" gas which causes the rate of gas flow through the turbine to remain in a range of values within which its response is relatively flat. This approach, however, only partially solves the problem. It does not actually eliminate variations in the turbine output with variations in the rate of flow therethrough. In any case, the use of bias gas flows is expensive since it requires the establishment of an additional accurately controlled gas flow, and since the gas flow paths through the instrument are more complex than they would be in the absence of a bias gas flow.